1. Field of the Invention
The invention relates generally to electromagnetically deflected beam display systems and more particularly to power supply control circuits for providing linear operation and high efficiency in random stroke and periodic raster display modes and during slew of a cathode ray tube electron beam.
2. Description of the Prior Art
The power efficiency of deflection systems that display both raster and stroke writing is relatively low due to the inductive deflection yoke and the high driving voltages required for magnetic deflection to assure adequate writing speed. Sophisticated airborne navigation displays with increased display area and information content require a significant increase in power consumption, while space and available power is limited. Since the deflection yoke driving circuit consumes a significant portion of the total display power, the power efficiency of the deflection system may be greatly enhanced if the required driving voltages can be reduced.
Since the rate of deflection for a raster display is generally much higher than for stroke deflection, the supply voltages applied for raster deflection are correspondingly higher. To obtain maximum slew speed during the stroke display also requires a relatively high supply voltage or reduced yoke inductance, both of which increase power dissipation of the system. However, during the writing phase of the stroke display, relatively low voltages may be satisfactory. Hence it is desirable to switch the power applied to the system to provide the minimum voltage required to assure linear operation.
One form of prior art apparatus providing dynamic power reduction was disclosed in U.S. Pat. No. 3,965,390, Power On Demand Beam Deflection System for CRT Displays, issued June 22, 1976 to James M. Spencer, Jr. and assigned to the assignee of the present invention. This invention utilized a flyback raster for retrace and provided reduced supply voltage only during the stroked deflection period when reduced writing speed was allowable.
An improved system is described in Ser. No. 858,149, Power As Required Beam Deflection System For CRT Displays With Raster Supply Switching, invented by W. W. Goldman and also assigned to the assignee of the present invention. Goldman provided an external raster/stroke control signal from a symbol generator to selectively apply a plurality of power supply sources to a push-pull yoke driver amplifier in accordance with the displayed mode of operation. Efficiency was further enhanced during raster operation by applying a control signal derived from the voltage developed across the yoke to synchronize the power switch closures. However, the limited voltage available during the stroke period resulted in inadequate high speed slewing capability. Further, it was desirable to eliminate the need for an external raster/stroke control signal inorder to minimize the complexity of the display circuitry.
The present invention describes a system for optimizing power conservation during the raster and stroke displays while permitting increased slewing speed. The invention is controlled by internal signals developed in the yoke driver amplifier without the need for external control signals. Since the internal switch control signals do not discriminate between stroke and raster operation, stroke writing efficiency is optimized even at high slewing speeds. Moreover, minimum power dissipation is also obtained during slewing conditions by varying the applied yoke driver voltages to that required to obtain linear operation.